This invention relates to apparatus and methods for automatically processing a plurality of liquid samples, with particular application to liquid chromatography.
The trend in modern liquid chromatography is towards higher resolution columns and higher sensitivity detectors. Samples with a volume of less than five microliters can now be processed to produce meaningful analytical results. With the capability to process small samples has come the demand for automated methods and apparatus for doing so.
Prior art autosamplers typically employ a circular tray capable of carrying a plurality of sample vials and a needle structure hydraulically connected to the input of a chromatography column. The sample vials are typically covered by septums to preserve the integrity of the samples. The needle is inserted into the vial, either by lifting the vial and impaling it on the needle, or by moving the needle assembly into the vial. When the needle is thus within the sample, the sample is withdrawn. This has been done either by aspirating the sample using a syringe, or by pressurizing the head space of the vial and forcing the liquid into the needle, or by combining pressurization and aspiration. Two needles, often concentrically configured, are sometimes used. The second needle is used either as a source of pressurized gas, or if aspiration is the sole means of sample withdrawal, to vent the head space to atmospheric pressure. The withdrawn sample may then be directed to a sample loop injection valve for injection into the liquid chromatograph. When handling very small samples one may only partially fill the sample loop to avoid any waste.
In order to efficiently utilize extremely small samples, it is necessary for the needle to extend to the very bottom of the vial. In addition, specially designed microvials having conical interiors are designed to handle such samples. Variations in vial designs, and in the manufacturing tolerances in a single vial design are such that it is very difficult to precisely control the location of the needle within the vial. If the needle is spaced apart from the bottom of the vial, valuable sample can be wasted. On the other hand, it is easy to break or bend a fragile sample needle by striking it on the bottom of the vial. One prior art solution to this problem employs vials having spring-loaded sample-containing inserts. When the needle engages the bottom of such a vial the entire insert moves downward minimizing the risk of needle breakage/bending. This approach involves a rather complex vial design with attendent high manufacturing costs.
Normally, it is necessary to wash the needle between sample withdrawals to avoid cross-contamination of samples. This is done by flowing a relatively large volume of wash solvent through the needle. The solvent used for this purpose becomes contaminated and must be disposed of as waste. In prior art autosamplers, waste solvent was disposed of into a special receptacle, or a particular carousel position was assigned to receive the waste. In the former case an additional mechanism was required in the autosampler, while in the latter case, the carousel must be rotated to the designated position resulting in time delays and considerable "busy-work" for the carousel.
Another trend in modern chromatographic analysis has been towards greater sample preparation prior to column injection. The need for precise control over the various sample preparation techniques is compounded when working with extremely small sample volumes. Until now most sample preparation has been done manually which is both less precise and more time-consuming than automated methods.
Accordingly, it is the object of the present invention to provide an autosampler mechanism for use in liquid chromatography which is capable of handling extremely small samples.
Another object of this invention is to provide a mechanism whereby the sample withdrawal needle will engage the bottom of the sample vial without risk of breaking or deforming the needle and without resort to complex vial designs.
Yet another object of this invention is to provide an autosampler with the ability to handle waste solvent from needle washing without resort to additional waste receptacle mechanisms or dedicating a carousel position to waste receipt.
Yet other objects of this invention will be apparent upon reading the following specification and claims.